a. The Nominative is formed from the stem by assing s in masculines and feminines, and m in neuters, the vowel o being weakened to u (see §§ 6. a, 46. N. 1).
b. In most nouns whose stem ends in ro- the s is not added in the Nominative, but o is lost, and e intrudes before r,[Compare the English chamber from French chambre.] if not already present: as, ager, stem agro-[Compare Greek agros, which shows the original o of the stem]; cf. puer, stem puero-.
c. The stem-vowel o has a variant form e,[By so-called Ablaut (see § 17. a).] which is preserved in the Latin vocative singular of nouns in -us: as, serve, vocative of servus, slave.
NOTE: In composition this e appears as i. This, - belli-ger, warlike (from belloe-, stem of bellum, war).